Radio repeater trunking (i.e., time sharing of a single repeater communications channel among many users) is well known. Although there are many actual potential applications for trunked radio repeater systems, one of the more important applications is for public service trunked (PST) systems. An exemplary PST system is discussed in commonly-assigned U.S. Pat. No. 4,905,302 to Childress et al entitled "Trunked Radio Repeater System," which is incorporated by reference herein.
As is well known, it may not be possible for a single RF repeater transmitting site to satisfactorily serve a geographically large coverage area. Accordingly, systems which must provide RF communications for an entire large geographical area (e.g., a major metropolitan area, a large county, etc.) typically include multiple RF transmissions sites. FIG. 1 is a schematic diagram of a simplified multiple-site "simulcast" system having three radio repeater (transmitting) sites S1, S2 and S3 for providing simulcast communications to geographical coverage areas A1, A2 and A3, respectively. A control point or "hub" C (e.g., a dispatch center) provides identical signals to each of sites S1 through S3 via links L1 through L3, respectively. These links are typically microwave links. Each site S1-S3 transmits the signals it receives from the control point C to its respective coverage area A, so that a mobile or portable transceiver receives the same signal no matter where it happens to be in the communications system overall coverage area A (which constitutes the "union", in an analogy to venn diagrams of the individual coverage areas A1, A2 and A3). In a simulcast system, the various transmitting sites transmit substantially the same signals substantially simultaneously at substantially the same radio frequency to avoid interference while increasing overall coverage areas.
Mobile or portable transceivers within area A1 can receive the signals transmitted by site S1, transceivers within area A2 can receive the signals transmitted by site S2, and transceivers within area A3 can receive signals by site S3. Transceivers moving out of a first site coverage area and into a second sites area cease monitoring the signals transmitted by the first site and begin monitoring the signals transmitted by the second site such that communication is continuously maintained without interruption so long as the transceiver stays within the overall combined system coverage area A'. An exemplary public service trunking simulcast system of this sort is disclosed in greater detail in commonly-assigned copending U.S. patent application Ser. No. 07/260,184 filed Oct. 20, 1988 entitled "Public Service Trunking Simulcast System" to Rose, Jr., and has been successfully in public use for quite some time.
Simulcasting in a multiple-site RF transmission is thus generally known. The following list (which is by no means exhaustive) of prior issued patents describe various aspects of RF transmission simulcasting and related PST issues:
U.S. Pat. No. 4,696,052 to Breeden; PA0 U.S. Pat. No. 4,696,051 to Breeden; PA0 U.S. Pat. No. 4,570,265 to Thro; PA0 U.S. Pat. No. 4,516,269 to Krinock; PA0 U.S. Pat. No. 4,475,246 to Batlivala et al; PA0 U.S. Pat. No. 4,317,220 to Martin; PA0 U.S. Pat. No. 4,972,410 to Cohen et al; PA0 U.S. Pat. No. 4,903,321 to Hall et al; PA0 U.S. Pat. No. 4,608,699 to Batlivala et al; PA0 U.S. Pat. No. 4,918,437 to Jasinski et al; PA0 U.S. Pat. No. 4,578,815 to Persinotti; PA0 U.S. Pat. No. 5,003,617 to Epsom et al; PA0 U.S. Pat. No. 4,939,746 to Childress; PA0 U.S. Pat. No. 4,903,262 to Dissosway et al; PA0 U.S. Pat. No. 4,926,496 to to Cole et al; PA0 U.S. Pat. No. 4,968,966 to Jasinski et al; PA0 U.S. Pat. No. 3,902,161 to Kiowski et al; PA0 U.S. Pat. No. 4,218,654 to Ogawa et al; PA0 U.S. Pat. No. 4,255,814 to Osborn; PA0 U.S. Pat. No. 4,411,007 to Rodman et al; PA0 U.S. Pat. No. 4,414,661 to Karlstrom; PA0 U.S. Pat. No. 4,472,802 to Pin et al; PA0 U.S. Pat. No. 4,597,105 to Freeburg; and PA0 Japanese Patent Disclosure No. 61-107826.
U.S. Pat. No. 4,903,321 to Hall et al, issued Feb. 20, 1990, entitled "Radio Trunking Fault Detection System," discloses a trunked radio repeater system having a radio frequency repeater site architecture that includes fault and call testing and failure detection features that are relevant to the presently preferred exemplary embodiment of the present invention. This patent is assigned to the assignee of the present invention and is incorporated by reference herein.
The present invention is directed toward improvements in a PST simulcast RF transmission system. In particular, the present invention is directed toward a method and apparatus for self correction of simulcast system timing. Specifically, in a PST simulcast system, the need for maintaining correct digital timing is extremely important since data timing is a very critical parameter in maintaining system performance. The communication paths (typically microwave links) between sites are subject momentary fades or "hits" on a somewhat unpredictable basis. Such fades or hits can cause equipment at the transmitting sites (e.g., modems) to lose synchronization with a common distributed system clocking signal--thereby destroying the time synchronicity critical to "simulcast" operation.
In general, maintaining data capability on the PST simulcast (in accordance with the above-mentioned copending Rose application) requires precise timing control. As such, the PST simulcast system described in accordance with the above-mentioned copending Rose patent application forces coherence at the start of data transmission on a particular established communications path, thus correcting for any multibit ambiguity created by the communication path modem. However, any "hit" or outage affecting a particular data path and its associated modems may cause the timing to be reestablished at a "random" latency. Since the timing and location of these "hits" is not predictable and, moreover, may occur remotely from the control point, it is not always practical to detect the timing fault, relay this information back to the control point, and then initiate effective action. In particular, there are three "hit" scenarios which must be handled: 1) a hit on a data communication path which causes a normal retrain of its associated modem using the data stream; 2) a hit on a data communication path which causes retrain of its associated modem using the data stream, before the data link is fully stable (which can result in abnormal latency); and 3) a hit on power "glitches" at a particular modem, causing it to default to its internal clock, rather than using the supplied external system clock.
Since data capability on the PST simulcast system requires precise timing control, the typical multiple bit ambiguity created by the modem must be corrected in order to provide proper system operation.
One approach to solving problems relating to a "type 1" hit (which approach has been in use in EGE systems since 1989) is to periodically reestablish the data timing assuming "normal" path conditions, on the working channels and on the control channels, in a manner which is transparent to the users and which provides preservation of interchannel timing as well. Resynchronization on a working channel may take place on a "per call" basis and resynchronization on the control channel may be implemented in a periodic (e.g., 54 second intervals) fashion.
More specifically, in its "idle" state, a preferred embodiment working channel (WC) station tristates a drive. As a resync will be initiated by a transmission of "1s" for more than 9 ms, by forcing the data line high during this tri state mode a resync can be initiated. Since this data drive is also tristated during the voice portion of a voice call, a resync will also be initiated there. This will effectively force coherence on (a) the first bit of every call (all calls begin with a digital message), and (b) the first bit of the channel drop message at the end of each call. If the channel gets reassigned before the entire channel sequence completes, the system changes the data context to reflect the channel coming backup. This is transferred to the resync action, and coherence, which was at the start of the drop is continued for the duration of the "channel coming up" message.
Considering a type 2 hit:
(a) The 600 Hz sync tone can be reduced to 300 Hz, effectively doubling the available range that can be corrected. (With an equalized channel, it is possible to go lower; or with analog FSK modems and voice channel, e.g., sampling jitter on "digital implemented" Bell 201 modem, makes them unacceptable.);
(b) The range of latency variation may be as much as 5 ms to 16 ms. A tone reference of approximately 50 Hz would be needed to cover this. (If 30 Hz was chosen, the FSL could be used directly and recover some errors completely.) (However, lowering the reference tone requires bigger FIFOs and also creates system latency.);
(c) A type 2 hit or event can be "fixed" by retraining the modem again once the communication path link is stable. This is analogous to resyncing for a type 1 hit, except that the retraining process is both longer and potentially more disruptive. Because a channel may be reassigned at any time, it is not feasible to retrain a modem following a test call. The only "benign" time for such an activity would be during a voice call. This requires that the "voice" time be at least 80 ms long to handle the channel drop message properly. (The control channel problem is more severe, as the length of retrain will not allow frame sync maintenance by the radios.)
Given that a type 2 event is rare, there is an alternative which is less disruptive. The preferred embodiment PST system has a test system (as described in the above-cited Hall et al patent) which periodically makes test calls to verify proper operation on each channel. As a type 2 hit will cause a test call failure (e.g., high speed data confirmation failure), the test call results can be used to initiate a modem retrain. The test call failure gets reported back to the channel "control" trunking card as an "inhibit" signal (e.g., through the alarm system). Since this inhibit signal already takes the channel out of service, the modem retrain is not disruptive. The TUAI ("test unit alarm interface") takes the test "call results" message and determines which channel to "take out of service" (all as described in the above-referenced Hall patent). The modems associated with the down channel may then be retrained (i.e., untrained and then retrained) without any disruptive effect on over-the-air signalling.
A type 3 hit can be handled by periodically instructing the modem to use its external (i.e., distributed common system) clock. This action is transparent, having no disruptive effect on modem operation. Because the timing errors result from the frequency difference in the two clocks (i.e., between the modem internal clock and the external system clock) a single extra or lost bit will typically occur every seven or eight seconds while using the internal clock. An acceptable "use external clock" instruction "kicking" rate was thus chosen to be approximately every thirty seconds (i.e., the modem is instructed by the kicker circuit to use the external clock at least every thirty seconds).
Thus, in accordance with one feature provided by the present invention, communication paths are restored to a "normal" state if they become "abnormal," thus allowing standard data timing to be reestablished. A "retraining" of the data modems used in the communications path is initiated upon the detection of a channel inhibit signal resulting from a test call failure. A channel inhibit signal is automatically generated in the preferred embodiment for a working channel from a "test" call (as discussed in the above mentioned commonly assigned Hall et al U.S. Pat. No. 4,903,321), which fails if the path timing is abnormal. More particularly, the preferred embodiment periodically checks proper transmitting station operation by making "test calls." Such test calls can fail due to improper system timing--thereby automatically taking the failed station out of service. In the preferred embodiment, modems for the failed station/channel are untrained and then retrained when the channel is so inhibited (thereby curing the problem if the problem resulted from the transmitting site modem losing synchronization with the central point modem). This approach is also transparent to the users (since the channel is inhibited anyway) and will quickly restore a communication data path to a "normal" timing state.
In accordance with another feature of the preferred exemplary embodiment of the present invention, data path modems are automatically restored to synchronization with the external distributed system clock in the event an aberration (e.g., a power glitch, outage, etc.) caused a modem to default to its own internal clock. To restore a communication path data modem back to using the external system clock, in accordance with a preferred embodiment of the present invention, a "kicker" circuit is provided to repetitively instruct the data modem to use the external clock. This approach quickly catches and restores proper operation regardless of what caused the modem to revert to its internal clock operation by default.
Thus, the present invention provides a means to protect against continuing system disruption following such events. The present invention also provides new and useful techniques and arrangements for reestablishing data timing to preserve system performance without disrupting normal system operation.